When a plurality of base materials are welded by arc welding, weaving welding is adopted in which welding is performed while advancing a welding electrode in the welding direction and weaving the welding electrode in a sinusoidal form in the right-left direction along a welding line. This weaving welding has hitherto been performed by swinging a welding torch itself in the right and left directions or tilting the welding electrode about the welding torch itself in the right and left directions. When such weaving welding is performed by an articulated robot, high trajectory accuracy is required.
In such an articulated robot, servo control is performed at each axis. However, since the natural frequency is low, for example, velocity feedforward control is rarely applied from the viewpoint of suppression of oscillation. Therefore, the phase delay of the actual feedback value with respect to the target value is large, and the response characteristic of a velocity controller in a servo control unit differs among the axes. This leads to a trajectory error. Since the position and velocity control characteristics (especially the phase delay characteristic) of the servo control unit change with the change in inertia resulting from the change in posture of the robot, nonlinear compensation control, which calculates, for example, the coupling torque between axes backward from the target value and makes compensation, has rarely effectively functioned owing to the phase shift. The following techniques are known for such trajectory control for the articulated robot.
Japanese Unexamined Patent Application Publication No. 10-217173 (PTL 1) discloses a decoupling controller for a multiaxis robot, which can obtain stable trajectory accuracy. This decoupling controller for the robot is provided in a multiaxis robot control device that controls a robot having a structure in which at least two arms are serially coupled via joints. This decoupling controller for the robot includes an inertial-matrix calculating mechanism that forms an inertial matrix from the angle of each arm and dynamic parameters such as mass and length of each arm, a computing mechanism that calculates decoupling condition variables including acceleration of each arm, and a computing mechanism that calculates decoupling gain to be multiplied by these condition variables. The decoupling controller further includes a multiplier and an adder that subject the decoupling condition variables and the decoupling gain to multiplication and addition, and a filter processor that eliminates higher harmonic ripples from a decoupling torque command obtained by these calculations.
Japanese Examined Patent Application Publication No. 8-7625 (PTL 2) discloses a position controller that enhances followability of a system to a trajectory command. This position controller includes one axis or a plurality of axes, and the axes are controlled by position feedback control. Herein, in addition to a position feedback loop, a feedforward loop is provided. In the feedforward loop, a position command is on the input side, a differentiation element having feedforward gain and an idle time element are provided in series, and velocity feedforward compensation output is added after position loop gain of the position feedback loop.